1. Field of the Invention
The present invention relates generally to a rotary-type actuator which is designed for rotatingly driving a member to be driven and compact for convenient installation. More particularly, the invention relates to a rotary actuator applicable for a variable damping force shock absorber assembly which is variable of damping characteristics.
2. Description of the Background Art
In the recent years, there have been developed various constructions of variable damping force shock absorber assemblies which is variable of damping characteristics for facilitating variable suspension characteristics in an automotive suspension system. Among such various constructions of variable damping force shock absorber assemblies, some of the shock absorber assemblies includes rotary valves to be rotatingly driven for varying flow resistance against working fluid in the shock absorbers and whereby adjusting damping characteristics. Such rotary valve-type variable damping force shock absorber assemblies have been disclosed in the U.S. Pat. No. 4,600,215, issued on July 15, 1986, to Kuroki et al, for example. In the shown construction, the rotary valve member defines a plurality of orifices respectively having different path areas for varying flow rate of the working fluid to flow between upper and lower fluid chambers in the shock absorber. The rotary valve member, as driven, varies angular position to establish fluid communication between the aforementioned upper and lower fluid chambers of the shock absorber through one of the orifices. With this construction, flow restriction magnitude for the working fluid is variable depending upon the angular position of the rotary valve member for varying the damping characteristics of the variable damping force shock absorber assembly.
On the other hand, in order to drive the rotary valve member for adjusting the damping characteristics, an electromagnetically operable actuator may be provided in the shock absorber assembly. One of the examples of such electromagnetically operable actuator has been disclosed in the Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 58-72546. The actuator disclosed in the above-identified Japanese Utility Model First Publication, comprises a stationary table fixed onto the top of a piston rod, an actuation rod drivingly connected to the rotary valve member, a rotor having permanent magnets and fixed to the actuation rod, and a stator which has a plurality of electromagnets. The electromagnets are arranged at positions radial outside of the permanent magnet and designed to be selectively energized for driving the rotor.
Such layout of the magnets in the actuator is so bulky to require substantial space for installation. This may raise incovenience to install the actuator on the top of the strut tower of the vehicular suspension, since it tends to interfere installation of other vehicular equipments.
On the other hand, in order to precisely adapt the damping characteristics of the shock absorber assembly to the vehicular driving condition or to obtain good response characteristics in adjustment of the damping characteristics, sufficient rotational torque of the actuator is required for quickly drive the actuation rod and the valve member, instantly. In order to obtain bigger torque, the area of the permanent magnet is to be expanded, the radial length of the permanent magnet is to be lengthened and/or the magnetic field of the electromagnet is strengthened. This means that greater torque to obtain may require larger size of the actuator to increase inconvenience in installation on the vehicle.